#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=110;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar();	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
LL h[maxn];
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL t;cin>>t;
  while(t--){
    LL n,k;cin>>n>>k;
    for(LL i=0;i<=n+10;i++) h[i]=0;
    for(LL i=1;i<=n;i++){
        cin>>h[i];
    }
    LL sum=0;LL pos=0;
    bool p=1;
    for(LL times=1;times<=k;times++){
        bool flag=1;
        for(LL i=1;i<=n;i++){
            if(h[i]<h[i+1]){
                sum++;
                pos=i;
                h[i]++;
                flag=0;
                break;
            }
        }
        if(flag==1){
            p=0;
            break;
        }
    }
    if(p==0){
        cout<<"-1"<<"\n";
    }
    else cout<<pos<<"\n";
  }
return 0;
}
